What is the best way to statically initialize an EnumMap in Java

Question

I need to statically initialize an EnumMap.I know two ways.

Using of() method of Map


private static final Map<<EnumKey>, <Value>> TEST_MAP = Map.of(ENUM_CONST1, "Value1", ENUM_CONST2, "Value2");

Using double brace initialization


private static final Map<<EnumKey>, <Value>> TEST_MAP = new EnumMap<>(EnumKey.class) {
    {
        put(ENUM_CONST1, "Value1");
        put(ENUM_CONST2, "Value2");
     }
};

Is there any other cleaner and more efficient way?

second one is a big anti-pattern and your first one does not initialize an `EnumMap`, but a `Map` (immutable one) — Eugene, May 19 '21 at 16:43

DuncG · Accepted Answer · 2021-05-20T12:06:21.003

A neat way to setup EnumMap is to define all the values in-line using Map.of or Map.ofEntries but note that this allocates a Map before the EnumMap constructor:

private static final EnumMap<YourEnum,String> A = new EnumMap<>(Map.of(
        YourEnum.ONE, "Value1",
        YourEnum.TWO, "Value2"
));

// Use Map.ofEntries for enums with more than 10 values:
private static final EnumMap<YourEnum,String> B = new EnumMap<>(Map.ofEntries(
        Map.entry(YourEnum.ONE, "Value1"),
        Map.entry(YourEnum.TWO, "Value2")
));

If wanting public access then wrap as unmodifiable Map (which is backed by EnumMap) or just pass back Map.of directly (but is not using an EnumMap).

public static final Map<YourEnum,String> C = Collections.unmodifiableMap(B);

score 2 · Answer 2 · answered May 19 '21 at 16:46

2

do it in a static block:

private static final EnumMap<....> yourMap = ...

static {
   yourMap.put(....);
   yourMap.put(....)
}

There will be ways to do this rather differently (but they don't exist in the API yet), via Constant Dynamic.

answered May 19 '21 at 16:46

Eugene

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any specific reason of doing it in static block? – sham May 19 '21 at 16:53
@sham how else? – Eugene May 19 '21 at 16:55
the first one also looks good. Is it better to do with static block – sham May 19 '21 at 17:00
Thanks got the answer. "Note that this Map.of method supports only a maximum of 10 key-value pairs." mention it in your article - https://www.baeldung.com/java-initialize-hashmap – sham May 19 '21 at 17:07
yeah agree...I was talking about `Map.of` which can store max of 10 mapping which is overcome with the `ofEntries` – sham May 19 '21 at 18:56
@sham A specific reason to use it could be to only have access to Java 8. Map.of() was introduced with Java 9. – user7427029 Oct 10 '22 at 12:29
hmm, static block or static method is the only way to do it in java 8 – sham Oct 11 '22 at 05:07

score 2 · Answer 3 · answered May 19 '21 at 17:28

2

Do it in a method:

private static final EnumMap<....> yourMap = yourMapMethod();

private static EnumMap<....> yourMapMethod() {
   EnumMap<....> yourMap = ...
   yourMap.put(....);
   yourMap.put(....);
   return yourMap;
}

answered May 19 '21 at 17:28

Andy Turner

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Thanks for sharing this approach. – javaguy May 20 '21 at 19:44

What is the best way to statically initialize an EnumMap in Java

3 Answers3